Functional Programming in React

Practical patterns for React apps. No jargon, just code that works.

Quick Reference

1. State with Option (Maybe It's There, Maybe Not)

Use Option instead of null | undefined for clearer intent.

Basic Pattern

import { useState } from 'react'
import * as O from 'fp-ts/Option'
import { pipe } from 'fp-ts/function'

interface User {
  id: string
  name: string
  email: string
}

function UserProfile() {
  // Option says "this might not exist yet"
  const [user, setUser] = useState<O.Option<User>>(O.none)

  const handleLogin = (userData: User) => {
    setUser(O.some(userData))
  }

  const handleLogout = () => {
    setUser(O.none)
  }

  return pipe(
    user,
    O.match(
      // When there's no user
      () => <button onClick={() => handleLogin({ id: '1', name: 'Alice', email: 'alice@example.com' })}>
        Log In
      </button>,
      // When there's a user
      (u) => (
        <div>
          <p>Welcome, {u.name}!</p>
          <button onClick={handleLogout}>Log Out</button>
        </div>
      )
    )
  )
}

Chaining Optional Values

import * as O from 'fp-ts/Option'
import { pipe } from 'fp-ts/function'

interface Profile {
  user: O.Option<{
    name: string
    settings: O.Option<{
      theme: string
    }>
  }>
}

function getTheme(profile: Profile): string {
  return pipe(
    profile.user,
    O.flatMap(u => u.settings),
    O.map(s => s.theme),
    O.getOrElse(() => 'light') // default
  )
}

2. Form Validation with Either

Either is perfect for validation: Left = errors, Right = valid data.

Simple Form Validation

import * as E from 'fp-ts/Either'
import * as A from 'fp-ts/Array'
import { pipe } from 'fp-ts/function'

// Validation functions return Either<ErrorMessage, ValidValue>
const validateEmail = (email: string): E.Either<string, string> =>
  email.includes('@')
    ? E.right(email)
    : E.left('Invalid email address')

const validatePassword = (password: string): E.Either<string, string> =>
  password.length >= 8
    ? E.right(password)
    : E.left('Password must be at least 8 characters')

const validateName = (name: string): E.Either<string, string> =>
  name.trim().length > 0
    ? E.right(name.trim())
    : E.left('Name is required')

Collecting All Errors (Not Just First One)

import * as E from 'fp-ts/Either'
import { sequenceS } from 'fp-ts/Apply'
import { getSemigroup } from 'fp-ts/NonEmptyArray'
import { pipe } from 'fp-ts/function'

// This collects ALL errors, not just the first one
const validateAll = sequenceS(E.getApplicativeValidation(getSemigroup<string>()))

interface SignupForm {
  name: string
  email: string
  password: string
}

interface ValidatedForm {
  name: string
  email: string
  password: string
}

function validateForm(form: SignupForm): E.Either<string[], ValidatedForm> {
  return pipe(
    validateAll({
      name: pipe(validateName(form.name), E.mapLeft(e => [e])),
      email: pipe(validateEmail(form.email), E.mapLeft(e => [e])),
      password: pipe(validatePassword(form.password), E.mapLeft(e => [e])),
    })
  )
}

// Usage in component
function SignupForm() {
  const [form, setForm] = useState({ name: '', email: '', password: '' })
  const [errors, setErrors] = useState<string[]>([])

  const handleSubmit = () => {
    pipe(
      validateForm(form),
      E.match(
        (errs) => setErrors(errs),     // Show all errors
        (valid) => {
          setErrors([])
          submitToServer(valid)         // Submit valid data
        }
      )
    )
  }

  return (
    <form onSubmit={e => { e.preventDefault(); handleSubmit() }}>
      <input
        value={form.name}
        onChange={e => setForm(f => ({ ...f, name: e.target.value }))}
        placeholder="Name"
      />
      <input
        value={form.email}
        onChange={e => setForm(f => ({ ...f, email: e.target.value }))}
        placeholder="Email"
      />
      <input
        type="password"
        value={form.password}
        onChange={e => setForm(f => ({ ...f, password: e.target.value }))}
        placeholder="Password"
      />

      {errors.length > 0 && (
        <ul style={{ color: 'red' }}>
          {errors.map((err, i) => <li key={i}>{err}</li>)}
        </ul>
      )}

      <button type="submit">Sign Up</button>
    </form>
  )
}

Field-Level Errors (Better UX)

type FieldErrors = Partial<Record<keyof SignupForm, string>>

function validateFormWithFieldErrors(form: SignupForm): E.Either<FieldErrors, ValidatedForm> {
  const errors: FieldErrors = {}

  pipe(validateName(form.name), E.mapLeft(e => { errors.name = e }))
  pipe(validateEmail(form.email), E.mapLeft(e => { errors.email = e }))
  pipe(validatePassword(form.password), E.mapLeft(e => { errors.password = e }))

  return Object.keys(errors).length > 0
    ? E.left(errors)
    : E.right({ name: form.name.trim(), email: form.email, password: form.password })
}

// In component
{errors.email && <span className="error">{errors.email}</span>}

3. Data Fetching with TaskEither

TaskEither = async operation that might fail. Perfect for API calls.

Basic Fetch Hook

import { useState, useEffect } from 'react'
import * as TE from 'fp-ts/TaskEither'
import * as E from 'fp-ts/Either'
import { pipe } from 'fp-ts/function'

// Wrap fetch in TaskEither
const fetchJson = <T>(url: string): TE.TaskEither<Error, T> =>
  TE.tryCatch(
    async () => {
      const res = await fetch(url)
      if (!res.ok) throw new Error(`HTTP ${res.status}`)
      return res.json()
    },
    (err) => err instanceof Error ? err : new Error(String(err))
  )

// Custom hook
function useFetch<T>(url: string) {
  const [data, setData] = useState<T | null>(null)
  const [error, setError] = useState<Error | null>(null)
  const [loading, setLoading] = useState(true)

  useEffect(() => {
    setLoading(true)
    setError(null)

    pipe(
      fetchJson<T>(url),
      TE.match(
        (err) => {
          setError(err)
          setLoading(false)
        },
        (result) => {
          setData(result)
          setLoading(false)
        }
      )
    )()
  }, [url])

  return { data, error, loading }
}

// Usage
function UserList() {
  const { data, error, loading } = useFetch<User[]>('/api/users')

  if (loading) return <div>Loading...</div>
  if (error) return <div>Error: {error.message}</div>
  return (
    <ul>
      {data?.map(user => <li key={user.id}>{user.name}</li>)}
    </ul>
  )
}

Chaining API Calls

// Fetch user, then fetch their posts
const fetchUserWithPosts = (userId: string) => pipe(
  fetchJson<User>(`/api/users/${userId}`),
  TE.flatMap(user => pipe(
    fetchJson<Post[]>(`/api/users/${userId}/posts`),
    TE.map(posts => ({ ...user, posts }))
  ))
)

Parallel API Calls

import { sequenceT } from 'fp-ts/Apply'

// Fetch multiple things at once
const fetchDashboardData = () => pipe(
  sequenceT(TE.ApplyPar)(
    fetchJson<User>('/api/user'),
    fetchJson<Stats>('/api/stats'),
    fetchJson<Notifications[]>('/api/notifications')
  ),
  TE.map(([user, stats, notifications]) => ({
    user,
    stats,
    notifications
  }))
)

4. RemoteData Pattern (The Right Way to Handle Async State)

Stop using { data, loading, error } booleans. Use a proper state machine.

The Pattern

// RemoteData has exactly 4 states - no impossible combinations
type RemoteData<E, A> =
  | { _tag: 'NotAsked' }                    // Haven't started yet
  | { _tag: 'Loading' }                     /